It's said that every senator wakes up in the morning, stares in the mirror, and sees a future president. But Sen. Jeff Flake (R-Ariz.), who is not running for re-election this year, may just look in the mirror and see a future spoiler, someone who mounts a doomed yet vital primary challenge to President Trump.

To that idea, I say: Go for it, senator.

Flake has become Trump's most incessant critic among elected Republicans, a position he is able to hold by virtue of the fact that he will no longer have to submit himself to the judgment of his party's primary voters. Liberated from that need, he has been sometimes blistering in his critiques of the president. "Our presidency has been debased. By a figure who seemingly has a bottomless appetite for destruction and division," he told Harvard Law School graduates last week. "We did not become great — and will never be great — by indulging and encouraging our very worst impulses. It doesn't matter how many red caps you sell."

That's a lot more blunt than the ordinary response Republicans make to the latest appalling thing Trump has said or done, which runs more toward "I don't condone that, but what's important is that we keep cutting taxes." So naturally, people are asking whether Flake is considering a challenge to Trump in 2020. When he was asked the question by Chuck Todd on Sunday's Meet the Press, here's how he responded:

It's not in my plans. But I've not ruled anything out. I do hope that somebody runs on the Republican side other than the president, if nothing else, simply to remind Republicans what conservatism is. And what Republicans have traditionally stood for. [Flake]

Flake is implicitly acknowledging that any challenge to an incumbent would be doomed from the start, which is almost certainly true. President Trump may be broadly unpopular, but among Republicans his approval stood at 87 percent in the most recent Gallup poll. History also tells us that challenges to incumbents don't succeed: In recent years, we've seen such challenges by Pat Buchanan (to George H.W. Bush in 1992), Ted Kennedy (to Jimmy Carter in 1980), and Ronald Reagan (to Gerald Ford in 1976). All of them failed.

There's something else about them, though: All of them saw that incumbent lose the general election. Which suggests that, if Flake really finds Trump as repugnant as he says, this would be the perfect way to attack him.

We shouldn't confuse correlation with causation. Those incumbents got primary challenges precisely because they were weak and stood a good chance of losing in the fall. Nevertheless, having a prominent primary opponent running around the country lobbing bombs at you while the other party does the same is the last thing an incumbent wants.

And for Jeff Flake, this could be an opportunity not just to stand up in opposition to Trump, but to actually help bring about his demise. Up until now Flake has been banging his head against the wall, getting a friendly reception from the media for his criticisms of Trump but not actually accomplishing much of anything. A primary challenge, however, would be an entirely different matter.

If Flake ran against Trump, he would get more attention for his ideas than he has ever imagined. He'd be on the news constantly. He might even get a small movement behind him, of traditional conservatives unhappy with Trump for one reason or another. In the best case scenario, Flake could imagine himself as the Republican Bernie Sanders, running a surprisingly successful insurgent campaign of ironclad principle against the choice of the establishment.

That might be a bit of a stretch, especially since Flake is about as establishment as they come. But if he wants to be heard, there's no better way than running for president.

On the other hand, if Flake thinks Trump's fans in his party hate him now for the occasional speech he delivers criticizing the president, just wait until he's actually undermining Trump's re-election. He'd be the target of the kind of venomous rage usually reserved for African-American athletes who stage silent protests.

But he should be able to endure it. And this is an opportunity that will never come again, a way to cap off his political career with a crusade that he plainly believes in, and that could make a real difference. In fact, it could be more consequential than anything he's done in his two decades as a politician.